There are several available mechanisms for retaining biomedical fasteners during orthopedic surgery. One mechanism is as simple as utilizing a slight taper on the driver and/or the screw itself, creating a taper lock. This method has at least two or three inherent disadvantages. If a taper is incorporated into the screw, the screw no longer has a “standard” interface and a non-standard driver may have to be used with it. When tapers are used on either the screw or the driver, very close manufacturing tolerances are required to achieve the desired functional results. Close manufacturing tolerances translate into higher manufacturing cost. Another problem with close manufacturing tolerances of a taper is the wear introduced into the driver through use. It only requires a small amount of wear to the tapered surfaces of the driver to cause a functional failure. Taper locks sometimes have a tendency to “lock” up too well, and so the user may have difficulty disengaging the driver from the screw.
Another retention method that is currently in use involves a plastic type material that is incorporated into the tip of the driver. The plastic material protrudes slightly proud of the mating surfaces of the driver, thus creating an interference fit compared to a slip fit between the male and female driver features. The plastic material is soft enough to flow or reshape itself, thereby allowing sufficient drag to overcome the force of gravity, retaining the screw to the driver tip. This design functions well when the driver is new. However, repeated usage wears out the plastic component and, as the friction reduces, the ability to retain the screw is lost. The plastic component also can fall out of the driver, which is a serious complication when the tool is being used during a surgical procedure. Furthermore, loss of the plastic component completely eliminates the retention function of the driver.
Another form of screw retention is based on a mechanical clip or retainer element. While this design strategy is fairly reliable, they too are subject to wear and eventual failure. The main obstacle in this form of a mechanism is its physical size. Typically, the clip or retainer is attached to the driver and will grasp the head of the screw in some fashion. The extra material at the working end of the driver may obstruct or limit visibility during placement of the screw in a surgery. Furthermore, this design is not necessarily compact enough to fit into the relative tight spaces involved in surgical procedures.
Other anchoring methods include variations of the three methods discussed above, which either create an interference (frictional drag), a surface to surface binding (taper lock), or a mechanical clip or retainer.
However, all of the existing retention mechanisms suffer from drawbacks as set forth above. There is therefore a need for a retention mechanism that does not suffer from the aforementioned drawbacks.